Bottle closures for alcoholic liquids, for example, wine, have historically been crafted of cork material. Cork is made from bark of certain trees, for example, the Cork Oak. Cork has qualities particularly suited to storing wine such as impermeability and a certain level of compressibility that allows for both a tight closure and removability. In contrast to bark, wood fibers do not have sufficient compressibility.
Due to extensive use, however, cork supplies are limited, thereby driving up price. Moreover, cork closures carry with them the risk of a taint that can be passed onto the wine. It has been estimated that as much as seven percent of wine bottles have some level of “corking”, or taint imparted by the cork.
Cork also has strength issues in applications using attached caps, where the cork is glued or otherwise fastened to the polymer, metal or wood top. Breakage rates are as high as 10% at some end users and the retail marketplace.
As a consequence, the beverage industry has sought other materials and structures for bottle closures. Metal “screw top caps” have been used with some success. Metal screw tops are formed of a metal skirt and plastic sealing layer. Screw tops extend over the outside of the bottle, as opposed to corks that are inserted into the bottle neck. While screw top caps are not susceptible to taint, screw top caps are sometimes questioned for their suitability for long term aging, and are unfavorably received by many consumers in this market. Moreover, screw top caps can lack aesthetic qualities associate with softer materials.
In other cases, it has been found that certain polymers can be used for wine bottle closures that behave in a manner more similar to cork. Polymer closures can have similar compressibility. However, polymer closures similarly suffer from a lack of aesthetics associated with fine wine. Polymer closures are also given to “creep”, which deforms the closure over time and can lead to failure.
Some attempts have been made to combine certain materials with the polymer closure to take advantage of the mechanical properties of the polymer while improving the aesthetics. In one example, a closure includes a plastic, wood or metal head portion glued to a thermoplastic polymer portion. The thermoplastic polymer portion inserts into the bottle, while the head remains outside the bottle and provides a gripping portion for extraction. The drawback of this design is that the glue joints often fail, causing separation of the polymer sealing material from the head.
Another common embodiment in this line of products is to mold a synthetic polymer shank over the top of another compatible polymer, such as polypropylene, where the two materials form a very strong cohesive bond. In this embodiment, the polymer shank material forms a very thick section to simulate natural cork and provide the necessary cushioning and tolerance zone in order to fit the variety of bottle necks found in the market. This type of closure construction is inefficient in its use of material and often results in voids, inclusions, size variations and other quality issues due to the thickness and volume of the molded material. This type of closure also is expensive to produce as the manufacturing cycle is relatively long and the volume of material required is large.
What is needed is a bottle closure that has sealing qualities comparable to cork, while having a versatile aesthetic human interface, and offering lower cost of manufacture.